The present embodiments relate to the deposition of phosphor materials in full colour ac thick film dielectric electroluminescent displays. More specifically, they are directed to a novel physical vapour deposition method for depositing thioaluminate thin film phosphor compositions using one or more source materials that comprise barium.
Thick film dielectric structures as described in U.S. Pat. No. 5,432,015 (the disclosure of which is incorporated herein by reference in its entirety) provide superior resistance to dielectric breakdown as well as a reduced operating voltage as compared to thin film electroluminescent (TFEL) displays. The thick film dielectric structure as deposited on a ceramic substrate will withstand somewhat higher processing temperatures than TFEL devices, which are typically fabricated on glass substrates. This increased high temperature tolerance facilitates annealing of phosphor films at higher temperatures to improve their luminosity. However, even with this enhancement, it is still desirable to improve display luminance and colour co-ordinates to keep pace with ongoing improvements to cathode ray tube (CRT) displays, particularly with recent trends in CRT specifications to higher luminance and higher colour temperature.
A high luminance blue-emitting electroluminescent phosphor is desirable in electroluminescent colour displays to achieve adequate luminosity. Cerium activated strontium sulfide has traditionally been selected as the blue light emitting phosphor material for full colour electroluminescent displays. However, the optical emission from this phosphor material must be passed through an appropriate chromatic filter to achieve the necessary colour co-ordinates for blue sub-pixels, resulting in a loss of luminance and energy efficiency. While cerium activated strontium sulfide phosphors have a relatively high energy conversion efficiency for blue emission of 1 lumen per watt, their spectral emission is quite wide ranging from blue to green necessitating the use of the optical filters. While the spectral emission of such phosphors can be shifted to some degree towards the blue by controlling the deposition conditions and activator concentration, it is not to the extent required to eliminate the need for an optical filter.
Alternative blue phosphor materials such as cerium activated alkaline earth thiogallate compounds have narrower emission spectra tuned to provide the colour co-ordinates required for blue sub-pixels. These compounds provide good blue colour co-ordinates, but have relatively poor luminosity and stability. Since the host materials are ternary compounds, it is relatively difficult to control the stoichiometry of the phosphor films. Europium activated barium thioaluminates provide excellent blue colour co-ordinates and higher luminance, but as a ternary compound, its stoichiometry is also somewhat difficult to control. Vacuum deposition of barium thioaluminate phosphor films comprising this material from a single sulfide source pellet using sputtering or electron beam evaporation has not yielded films with adequately high luminosity.
Improved luminance of barium thioaluminate phosphors has been achieved by using a hopping electron beam deposition technique to deposit films from two source pellets, one comprising barium sulfide doped with europium and the other comprising aluminum sulfide. The stoichiometry of the deposited film is controlled by controlling the relative dwell time of the electron beam impinging on each of the two source materials. However, this technique is not readily scalable to facilitate commercial production of large area displays and the process cannot be adequately controlled to compensate for changes in the evaporation rates from the two sources as the deposition proceeds and the source pellets are depleted.
The stoichiometry of thioaluminate phosphors can be improved using more than one electron beam impinging on each source for the deposition. This approach requires added controls over the relative deposition rates for the different sources. Furthermore, the required relative evaporation rates must be calibrated for each specific piece of deposition equipment and the requirement for multiple sources constrains the design of the deposition equipment, which generally adds to the cost of the equipment. Lastly, certain known evaporation methods are not well suited for the deposition of large area films such as a required for the fabrication of large electronic displays such as those for the wall television application.
U.S. Pat. No. 6,447,654 discloses the sputtering of barium thioaluminate phosphor films from a single target comprising aluminum sulfide and barium sulfide to deposit blue-emitting barium magnesium thioaluminate phosphor materials. The stoichiomentry of the deposited film is adjusted by adjusting the target composition to account for differential condensation rates of the target elements on the phosphor film substrate. However, this method does not fully solve the problem of providing a stable phosphor film during display operation and at the same time providing a method that can be used for the economic deposition of phosphor films over large areas.
The Applicant's co-pending U.S. patent application Ser. No. 10/036,559 discloses the sputtering of two targets to deposit a rare earth activated barium thioaluminate phosphor film. One of the sputtering targets comprises aluminum while the other sputtering target comprises europium doped barium sulfide. The sputtering is carried out in a low pressure atmosphere of hydrogen sulfide to provide sufficient sulfur content in the deposited film. The use of two sputtering targets facilitates modulation of the relative deposition rate of materials arising from each source which in turn facilitates deposition of a laminated film with a periodic composition alternately rich and poor in aluminum. The variation can be achieved by using a rotating or oscillating substrate that is alternately positioned in the flux of atomic species sputtered from the respective targets.
To the extent that the atomic flux from the two sources are spatially separated from one another, and to the extent that hydrogen sulfide is present in the sputtering chamber, a film can be deposited with a composition that is alternately aluminum sulfide and rare earth doped barium sulfide. The thickness of the layers can be altered by changing the rotation rate or the oscillation rate of the substrate. In this method, however, the composition modulation across the thickness of the deposited layer is problematical for subsequent reaction of the deposited materials to form a homogeneous single phase phosphor material, since atomic species are required to diffuse within the deposited film to achieve a homogeneous composition on an atomic scale.
It is therefore desirable to develop an efficient method for the deposition of thin film phosphor compositions for thick film dielectric electroluminescent displays that obviates one or more of the disadvantages of the prior art methods.